This invention relates to an improved fuel composition and fuel additives which are useful as cloud point depressants.
Distillate fuels such as diesel fuels tend to exhibit reduced flow at reduced temperatures. This reduced flow affects the transport and use of the distillate fuels not only in the refinery but also in an internal combustion engine. If the distillate fuel is cooled to below a temperature at which solid formation begins to occur in the fuel, generally known as the cloud point (ASTM D 2500) or wax appearance point (ASTM D 3117), solids forming in the fuel will essentially prevent the flow of the fuel, plugging piping in the refinery, during transport of the fuel, and in inlet lines supplying an engine. Under low temperature conditions during consumption of the distillate fuel, as in a diesel engine, wax precipitation and gelation can cause the engine fuel filter to plug.
As used herein, distillate fuels encompass a range of fuel types, typically including but not limited to kerosene, intermediate distillates, lower volatility distillate gas oils, and higher viscosity distillates. Grades encompassed by the term include Grades No. 1-D, 2-D and 4-D for diesel fuels as defined in ASTM D 975, incorporated herein by reference. The distillate fuels are useful in a range of applications, including use in automotive diesel engines and in non-automotive applications under both varying and relatively constant speed and load conditions.
The cloud point of a fuel is the temperature at which a cloud of wax crystals first appears in a liquid when it is cooled under conditions prescribed in the test method as defined in ASTM D 2500, incorporated herein by reference. The cloud point behavior of a distillate fuel such as diesel fuel is a function of its composition. The fuel is comprised of a mixture of hydrocarbons including normal paraffins, branched paraffins, olefins, aromatics and other non-polar and polar compounds. As the diesel fuel temperature decreases at the refinery, during transport, or in a vehicle, one or more components of the fuel will tend to separate, or precipitate. The cloud point of the fuel is defined as the temperature at which the first waxes appear. The cloud point corresponds to an equilibrium state based on thermodynamic relationships which determine the solubility of paraffins in the diesel fuel.
Additives to decrease the cloud point, also known as cloud point depressants, have been used in fuels to delay the formation of solid wax crystals and thereby aid in enhancing the operability of the fuel. In addition, a cloud point depressant may also provide economic benefits in connection with the refining of the diesel fuel. To reach a particular cloud point specification, a certain amount of hydrocarbons in the kerosene boiling range are left in the diesel fuel fraction. A cloud point depressant will typically lower the cloud point by 2 to 3xc2x0 C. This lowering of the cloud point temperature by the depressant is known to compensate for the backing out of 20 to 30% of the kerosene fraction originally required to meet the cloud point specification.
The components of the diesel fuel having the lowest solubility tend to be the first to separate as solids from the fuel with decreasing temperature. Straight chain hydrocarbons, such as normal paraffins, generally have the lowest solubility in the diesel fuel. Generally, the paraffin crystals which separate from the diesel fuel appear as individual crystals. As more crystals form in the fuel, they tend to agglomerate and eventually reach a particle size which becomes visible to the eye and creates a cloudy appearance.
It is known to incorporate additives into diesel fuel to enhance the flow properties of the fuel at low temperatures. These additives are generally viewed as operating under either or both of two primary mechanisms. In the first, the additive molecules have a configuration which allow them to interact with the n-paraffin molecules at the growing ends of the paraffin crystals. The interacting additive molecules by steric effects act as a cap to prevent additional paraffin molecules from adding to the crystal, thereby limiting the length of the existing crystal.
In the second mechanism, the flow modifying additive may improve the flow properties of diesel fuel at low temperatures by functioning as a nucleator to promote the growth of smaller size crystals.
Additional, secondary, mechanisms involving the modification of wax properties in the fuel by incorporation of additives include, but are not limited to, dispersal of the wax in the fuel and solubilization of the wax in the fuel.
The range of available diesel fuels includes Grade No. 2-D, defined in ASTM D 975-90 as a general purpose, middle distillate fuel for automotive diesel engines, which is also suitable for use in non-automotive applications, especially in conditions of frequently varying speed and load. Certain of these Grade No. 2-D (No. 2) fuels may be classified as being hard to treat when using one or more additives to improve flow. A hard-to-treat diesel fuel is either unresponsive to a flow improving additive, or requires increased levels of one or more additives relative to a normal fuel to effect flow improvement.
Fuels in general, and diesel fuels in particular, are mixtures of hydrocarbons of different chemical types (i.e., paraffins, aromatics, olefins, etc.) wherein each type may be present in a range of molecular weights and carbon lengths. The cloud point temperature is a function of one or more properties of the fuel, the properties being attributable to the composition of the fuel. These properties include the paraffin concentration of the fuel, the molecular weight of the paraffins, and the chemical nature of the non-paraffin part of the fuel. For example, in the case of a hard-to-treat fuel the compositional properties which render a fuel hard to treat relative to normal fuels include a narrower wax distribution; the virtual absence of very high molecular weight waxes, or inordinately large amounts of very high molecular weight waxes; a higher total percentage of wax; and a higher average normal paraffin carbon number range. It is difficult to generate a single set of quantitative parameters which define a hard-to-treat fuel. Nevertheless, measured parameters which tend to identify a hard-to-treat middle distillate fuel include a temperature range of less than 100xc2x0 C. between the 20% distilled and 90% distilled temperatures (as determined by test method ASTM D 86 incorporated herein by reference), a temperature range less than 25xc2x0 C. between the 90% distilled temperature and the final boiling point (see ASTM D 86), and a final boiling point above or below the temperature range 360xc2x0 to 380xc2x0 C.
A fuel will cool to its cloud point generally in a static environment, but will also become cloudy in a dynamic environment such as a moving fuel tank at sufficiently low temperature. There continues to be a demand for additives which improve the cloud point characteristics of distillate fuels. Because additives are incorporated into the fuel to improve distinct characteristics of the fuel, it is possible that one additive may have an antagonistic effect on another additive. It is therefore desired that the cloud point additive not demonstrate an antagonism to the characteristics of the fuel as to one or more other properties, such as cold flow or wax anti-settling properties. Further, there remains a need for additive compositions which are capable of depressing the cloud point of hard-to-treat fuels.
It has been found that certain polyimide and maleic anhydride olefin polymer additives with carbon substituent chain lengths within a specified range, and alternatively certain ethyl vinyl acetate isobutylene terpolymers will depress the cloud point of certain distillate fuels such as No. 2 diesel fuel. Also, the above polyimide and maleic anhydride olefin polymer additives in combination with other materials such as ethylene vinyl acetate isobutylene terpolymers demonstrate substantial improvement in depressing the cloud point of certain distillate fuels when incorporated therein. With the latter additive combinations the cold flow properties of the distillate fuels are not adversely affected by the incorporation of the polyimide or maleic anhydride olefin polymer additive.
Copending application Ser. No. 09/311,465 is directed to certain maleic anhydride xcex1-olefin copolymer and polyimide additives incorporated into distillate fuel to improve the wax anti-settling properties of the fuel. Copending application Ser. No.09/311,459 is directed to the combination of an ethylene vinyl acetate isobutylene terpolymer with one or more additive components including certain maleic anhydride xcex1-olefin copolymer and polyimide components to effect cold flow improvement in distillate fuels.
The maleic anhydride olefin copolymer additive is prepared by the reaction of maleic anhydride with xcex1-olefin. Generally this copolymer additive contains substantially equimolar amounts of maleic anhydride and xcex1-olefin. The operative starting xcex1-olefin is a mixture of individual xcex1-olefins having a range of carbon numbers. The starting xcex1-olefin composition used to prepare the maleic anhydride olefin copolymer additive of the invention has at least a minimum xcex1-olefin concentration by weight with a carbon number within the range from about C16 to about C18. The additive generally does not contain xcex1-olefin of a single carbon number; thus the additive consists of blends of xcex1-olefins having carbon numbers within this range. The operative starting xcex1-olefin may have a minor component portion which is outside the above carbon number range. The maleic anhydride xcex1-olefin copolymers have a number average molecular weight in the range of about 700 to about 10,000 as measured by vapor pressure osmometry.
The invention also encompasses a cloud point depressant comprising a polyimide produced by the reaction of an alkyl amine, maleic anhydride and xcex1-olefin. Generally the polyimide is produced from substantially equimolar amounts of maleic anhydride and xcex1-olefin. The operative xcex1-olefin is similar in composition to that described above for the maleic anhydride olefin copolymer additive, having a carbon number range from about C16 to about C18. Particularly advantageous cloud point depressant properties are obtained when the alkyl amine is tallow amine. The polyimide has a number average molecular weight in the range of about 1,200 to about 10,000, preferably in the range of about 1,200 to about 5,000, as measured by vapor pressure osmometry.
The ethylene vinyl acetate isobutylene terpolymer additive has a weight average molecular weight in the range of about 1,500 to about 18,000, preferably about 3,000 to about 12,000; a number average molecular weight in the range of about 400 to about 3,000, preferably about 1,500 to about 2,500; and a ratio of weight average molecular weight to number average molecular weight from about 1.5 to about 6.
It has been found that unexpectedly advantageous cloud point depressing properties can be imparted to distillate fuels by incorporating an additive having the following structure: 
wherein R has at least 80% by weight of a hydrocarbon substituent from about 14 to about 16 carbons, and n is from about 2 to about 30. Preferably R has at least 90% by weight of a hydrocarbon substituent from about 14 to about 16 carbons, and most preferably R has at least 95% by weight of a hydrocarbon substrate from about 14 to about 16 carbons. The resulting maleic anhydride xcex1-olefin copolymer has a number average molecular weight in the range of about 700 to about 10,000, and preferably in the range of about 700 to about 4,000, as determined by vapor pressure osmometry.
The cloud point depressant additive of this invention typically encompasses a mixture of hydrocarbon substituents of varying carbon number within the recited range, and encompasses straight and branched chain moieties.
It has also been found that an additive of the structure 
wherein R has at least 80% by weight of a hydrocarbon substituent from about 14 to about 16 carbons, Rxe2x80x2 has at least 80% by weight of a hydrocarbon substituent from 16 to 18 carbons, and n is from about 2 to about 17, also has cloud point depressant properties. Preferably R has at least 90% by weight of a hydrocarbon substituent from about 14 to about 16 carbons, and most preferably R has at least 95% by weight of a hydrocarbon substituent from about 14 to about 16 carbons. Typically, Rxe2x80x2 has at least 90% by weight of a hydrocarbon substituent from 16 to 18 carbons. The above additive, described as a polyimide, has a number average molecular weight as determined by vapor pressure osmometry in the range of about 1,200 to about 10,000, and preferably in the range of about 1,200 to about 5,000.
In addition, it has been found that certain ethylene vinyl acetate isobutylene terpolymers demonstrate cloud point depressant properties both alone and in combination with one or more of the above maleic anhydride xcex1-olefin copolymer or polyimide additives. Useful ethylene vinyl acetate isobutylene terpolymers have a weight average molecular weight in the range of about 1,500 to about 18,000, a number average molecular weight in the range of about 400 to about 3,000, and a ratio of weight average molecular weight to number average molecular weight from about 1.5 to about 6. Preferably the weight average molecular weight ranges from about 3,000 to about 12,000, and the number average molecular weight ranges from about 1,500 to about 2,500. The terpolymers have a Brookfield viscosity in the range of about 100 to about 300 centipoise at 140xc2x0 C. Typically the Brookfield viscosity is in the range of about 100 to about 200 centipoise. Vinyl acetate content is from about 25 to about 55 weight percent. Preferably the vinyl acetate content ranges from about 30 to about 45 weight percent; more preferably the vinyl acetate content ranges from about 35 to about 45 weight percent. The branching index is from 2 to 15, and preferably 5 to 10. The rate of isobutylene introduction depends on the rate of vinyl acetate introduction, and may range from about 0.01 to about 10 times the rate of vinyl acetate monomer flow rate to the reactor.
A fuel will cool to its cloud point generally in a static environment, such as storage tanks, shipping tanks or even fuel tanks where no separate agitation is supplied. However, a fuel will become cloudy even in a dynamic environment such as a moving fuel tank at sufficiently low temperature. To replicate the conditions which promote formation of a cloud point and permit evaluation of additives, ASTM D 2500 for measuring cloud point formation or ASTM D 3117 for measuring the wax appearance point, both incorporated herein by reference, can be utilized.
Optionally, the maleic anhydride xcex1-olefin copolymer or polyimide can be combined with an ethylene vinyl acetate isobutylene terpolymer or ethylene vinyl acetate copolymer to produce a cloud point depressant additive combination which also provides cold flow improvement without adversely affecting the cloud point depressant properties.
The maleic anhydride xcex1-olefin copolymer or polyimide additives of the present invention act as cloud point depressants when effective amounts are added to distillate fuels. Useful amounts of the additives range from about 50 to about 1,500 ppm by weight of the fuel being treated. Preferred amounts of the additives to improve cloud point depressant properties range from about 250 to about 500 ppm by weight of treated fuel. Maleic anhydride xcex1-olefin copolymers and polyimides used according to the teachings of this invention may be derived from xcex1-olefin products such as those manufactured by Chevron Corporation and identified as Gulftene(copyright) 18 Alpha-Olefin, or the like.
Useful amounts of the terpolymers range from about 10 to about 1,000 ppm by weight of the fuel being treated. Preferred amounts of terpolymers range from about 25 to about 250 ppm by weight of treated fuel in connection with improving cloud point depression.
The additives of this invention may be used as the sole additive in a distillate fuel. Also, the polyimide or maleic anhydride xcex1-olefin copolymers may be used in combination with one or more terpolymers or copolymers as described above. In addition, cloud point depressant additives of this invention may be used in combination with other fuel additives such as corrosion inhibitors, antioxidants, sludge inhibitors, cold flow improvers, wax anti-settling agents, and the like.